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Nitrogen cycling processes can be tracked using quantitative Polymerase Chain Reaction (qPCR) to determine the presence and qReverse Transcriptase-PCR (qRT-PCR) to determine expression of key genes, or ‘biological markers’, for nitrogen metabolism. Nitrification is catalyzed in part, by two enzymes: ammonia monooxygenase (AMO; NH3 NH2OH) and nitrite oxidoreductase (NXR; NO2- NO3-). For denitrification, four enzymes act sequentially: nitrate reductase (NAR/NAP; NO3- NO2-), nitrite reductase (NIR; NO2- NO), nitric oxide reductase (NOR; NO N2O), and nitrous oxide reductase (NOS; N2O N2). A principle of wastewater treatment (WWT) is to remove excess nitrogen by taking advantage of natural nitrogen cycling or biological nitrogen removal (BNR). This process involves using microorganisms to bring influent ammonia through nitrification and denitrification to release nitrogen gas, which does not contribute to eutrophication. A novel shortcut nitrogen removal configuration could increase nitrogen removal efficiency by promoting nitritation/denitritation, reducing the classic nitrogen cycle by removing the redundant oxidation/reduction step to nitrate (NO3-). Here, three nitrogen transformations were used to track the three main phases in the nitrogen cycle; ammonia monooxygenase for nitrification, nitrite oxidoreductase for shortcut, and nitrous oxide reductase for denitrification. Primers for qPCR and qRT-PCR were designed to capture as much sequence diversity as possible for each step. Genes from bacteria known to perform the nitrogen transformations of interest (amoA, nxrB, nosZ) were used to BLAST-query the Integrated Microbial Genomes & Microbiomes database (img.jgi.doe.gov) to find homologs from organisms commonly found in WWT. These sequences were then aligned to find regions sufficiently conserved for primer design. These PCR primers were tested against standards for each gene and used to track nitrogen transformation potential and expression in a novel lab-scale algal photo-sequencing batch reactor which promotes shortcut nitrogen removal from wastewater across three solids retention times (SRT, or mean cell residence time); 5, 10 and 15 days. SRT 15 had the greatest total nitrogen removal with nitritation and denitritation observed. Nitrate was not detected in the first cycle and shortcut nitrogen removal was supported by low levels of nxrB genes and transcripts. Simultaneous nitrification/denitrification was supported by elevated concentrations of nosZ during the light period and less nitrite produced than ammonium consumed. Nitritation was predominantly performed by Betaproteobacteria amoA and nitrous oxide reduction was predominantly from nosZ group I (Proteobacteria-type). |
